1. Field of the Invention
The present invention relates to a semiconductor device using crystalline semiconductor, and to a process for fabricating the same.
2. Prior Art
Thin film transistors (hereinafter referred to simply as xe2x80x9cTFTsxe2x80x9d) are well known as devices utilizing thin film semiconductors. The TFTs are fabricated by forming a thin film semiconductor on a substrate and processing the thin film semiconductor thereafter. The TFTs are used in various types of integrated circuits, and are particularly noted in the field of electrooptical devices; especially, they attracted much attention in the field of switching elements that are provided to each of the pixels of active matrix (addressed) liquid crystal display devices and driver elements of the peripheral circuits thereof.
Amorphous silicon film can be utilized most readily as the thin film semiconductors for TFTs. However, the electric characteristics of the amorphous silicon film are disadvantageously poor. The use of a thin film of crystalline silicon can solve the problem. Crystalline silicon films are known as polycrystalline silicon, polysilicon, microcrystalline silicon, etc. The crystalline silicon film can be prepared by first forming an amorphous silicon film, and then heat treating the resulting film for crystallization.
However, the heat treatment for the crystallization of the amorphous silicon film requires heating the film at a temperature of 600xc2x0 C. or higher for 20 hours or longer. Such a heat treatment has a problem that it is difficult to use a glass as a substrate. For instance, a Corning 7059 glass commonly used for the substrate of active matrix liquid crystal display devices has a glass deformation temperature of 593xc2x0 C., and is therefore not suitable for using in large area substrates that are subjected to heating at a temperature of 600xc2x0 C. or higher. That is, if a commonly used Corning 7059 glass substrate is heated at a temperature of 600xc2x0 C. or higher for 20 hours or longer, distinct shrinking and warping occur on the substrate.
The aforementioned problem can be overcome by performing the heat treatment at a temperature as low as possible. On the other hand, from the viewpoint of increasing productivity, it is required to shorten the duration of this heat treatment step as much as possible.
In case of crystallizing an amorphous silicon film by heating, moreover, the entire film becomes crystalline. Accordingly, it is not possible to crystallize the film locally, or to control the crystallinity of a particular region.
To overcome the aforementioned problems, JP-A-2-140915 and JP-A-2-260524 (the term xe2x80x9cJP-A-xe2x80x9d as referred herein signifies an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) propose a technique which comprises artificially introducing portions or regions inside an amorphous silicon film to provide sites as the crystallization nuclei, and then heat treating the amorphous silicon film thereafter to crystallize the film selectively. The technology allows crystal nuclei to form at predetermined sites within the amorphous silicon film.
According to the constitution of JP-A-2-140915, for instance, an aluminum layer is formed on the amorphous silicon film, and crystal nuclei are allowed to generate in the portion at which the amorphous silicon is in contact with aluminum. Thus, by heat treating the resulting film, crystal growth is initiated from the thus provided crystal nuclei. JP-A-2-260524 proposes a constitution which comprises adding tin (Sn) into an amorphous silicon film by means of ion implantation, and then generating crystal nuclei from the region into which tin ions are added.
Aluminum (Al) and tin (Sn) are substitutive metallic elements. Thus, they cannot diffuse and intrude deeply into the silicon film because they readily form an alloy with silicon. Accordingly, the alloy serves as the crystal nuclei, and the crystallization in this case proceeds from these alloy portions. The crystallization process using Al or Sn is characterized in that the crystal growth occurs from the portion into which Al or Sn is introduced (i.e., the alloy layer of Si and Al or Sn). In general, crystallization proceeds in two steps; a first step of generating initial nuclei, and a subsequent step of crystal growth which occurs from the initial nucleation sites. The metallic elements of the substitutive type, i.e., Al and Sn, are effective for generating the initial nucleation sites, but have almost no effect on the later step of crystal growth.
Thus, the temperature of crystallization cannot be lowered nor the duration of crystallization be shortened by using Al or Sn. That is, the method using Al or Sn is none the better as compared with the conventional crystallization of simply heating the amorphous silicon film.
According to the study of the present inventors, it is possible to crystallize an amorphous silicon film by heating the film at 550xc2x0 C. for about 4 hours. This can be accomplished by first depositing a trace amount of an intrusion type element, such as nickel or palladium, on the surface of the amorphous silicon film, and heating the resulting product thereafter. The intrusion type metallic elements not only facilitates the initial nucleation, but also accelerates the later crystal growth. Thus, as compared with a conventional process which comprises simply heating the film, the heating temperature can be lowered, and the duration of heating can be shortened.
The elements above (i.e., catalyst elements which accelerate the crystallization) can be introduced into the amorphous silicon film in a trace quantity by means of plasma treatment, vapor deposition, or ion implantation. Plasma treatment as referred herein signifies adding the catalyst elements into the amorphous silicon film by effecting the treatment in a plasma CVD apparatus of a parallel plate type or a positive column type by using a material containing the catalyst element as the electrode, and allowing a plasma to generate under, for instance, gaseous nitrogen or gaseous hydrogen.
Metallic elements which accelerate the crystallization above are the intrusive elements such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, and Au. These intrusive elements diffuse into the silicon film during the heat treatment step. The crystallization proceeds with progressive diffusion of these intrusive elements. That is, the intrusive elements accelerate the crystallization of the amorphous silicon film by exerting catalytic function at every site they proceed inside the amorphous silicon film.
Thus, by incorporating the intrusive elements to the amorphous silicon film, crystallization can be accelerated in a manner differing from that which proceeds gradually from the crystal nuclei. For instance, if an intrusive element is introduced into a particular portion of an amorphous silicon film and is subjected to heat treatment thereafter, crystallization proceeds from the site the metallic element was introduced, and in a direction parallel to the surface of the film. The length of the thus crystallized region amounts to several tens of micrometers or even longer. Furthermore, by introducing the metallic element above to the entire surface of the film, the film can be wholly and yet uniformly crystallized. As a matter of course, the entire film exhibits a polycrystalline or a microcrystalline structure, but the grain boundary thereof is not distinguished. Accordingly, a device having stability in characteristics can be obtained by using the desired portion of the film.
The intrusive elements above rapidly diffuse into the silicon film. Accordingly, the key of this method is the estimation of the quantity to be introduced (added). If the elements are introduced insufficiently, the addition of the elements results in a small effect. A film with favorable crystallinity cannot be expected in such a case. If the elements are introduced in an excessive quantity, on the other hand, the semiconductive characteristics of silicon would be impaired.
Accordingly, the optimum quantity of the intrusive metallic elements above must be estimated. For instance, Ni effectively accelerates the crystallization if it is added into the crystallized silicon film at a concentration of 1xc3x971013 cmxe2x88x923 or higher. So long as the concentration of nickel does not exceed 1xc3x971013 cmxe2x88x923, the semiconductive characteristics of the silicon film remains without being impaired. The concentration of the elements in this case is defined as the minimum value obtainable by SIMS (secondary ion mass spectroscopy).
The above description applies not only to nickel, but also to the metallic elements other than nickel enumerated above. Thus, the same effect is expected on the other metallic elements so long as they are added at a concentration in a range defined above.
To control the concentration of the metallic elements above (those metallic elements capable of accelerating crystallization are referred to hereinafter as xe2x80x9ccatalyst elementsxe2x80x9d) inclusive of nickel in an optimum range for accelerating crystallization, the quantity thereof must be controlled at the point of their introduction.
Considering a case of using nickel as the catalyst element, an amorphous silicon film was deposited, and nickel was added therein by plasma treatment to fabricate a crystalline silicon film. The progress of crystallization and the like was studied in detail. The following points were found as a result:
(1) In case nickel is introduced into the amorphous silicon film by means of plasma treatment, nickel has already been intruded into the amorphous silicon film to a considerable depth before subjecting the film to heat treatment;
(2) Initial crystal nucleation occurs from the surface at which nickel was introduced; and
(3) In case of depositing nickel on an amorphous silicon film by evaporation, crystallization occurs in a manner similar to that occurred in plasma treatment.
It can be found from the above findings that not all nickel introduced by plasma treatment function sufficiently effective. That is, even if nickel is introduced in a large quantity, it does not follow that all the nickel atoms function sufficiently. It is therefore assumed that the point (or plane) at which nickel contacts silicon functions to decrease the temperature of crystallization. Conclusively, nickel atoms are preferably dispersed as finely as possible in the amorphous silicon film. In other words, xe2x80x9cnickel atoms need to be introduced in the vicinity of the surface of amorphous silicon film at a minimum concentration necessary for realizing the low temperature crystallization of the amorphous silicon filmxe2x80x9d.
Vapor deposition can be mentioned as a method for introducing nickel atoms in a trace amount in only the vicinity of the surface of the amorphous silicon film, i.e., introducing catalyst elements capable of accelerating the crystallization in a trace amount in only the vicinity of the surface of the amorphous silicon film. However, vapor deposition is inferior concerning its controllability, and it fails to precisely control the quantity of catalyst element to be introduced in the amorphous silicon film.
Furthermore, it is required to minimize the quantity of the catalyst element to a level as low as possible. However, there is still a problem that the crystallinity becomes an impurity.
In the light of the aforementioned circumstances, an object of the present invention is to provide a process for fabricating a thin film of crystalline silicon semiconductor by performing heat treatment at 600xc2x0 C. or lower and using a catalyst element, yet characterized in that:
(1) the catalyst elements are introduced at a minimum and controlled amount;
(2) the process yields high productivity; and
(3) the product thus obtained exhibits a higher crystallinity as compared with the prior art silicon semiconductors fabricated by heat treatment.
The above object of the present invention can be achieved by a process for fabricating a crystalline silicon described below.
That is, the process comprises: maintaining in contact with an amorphous silicon film, a compound containing an element of a catalyst which accelerates the crystallization of amorphous silicon or a compound containing the same; crystallizing the amorphous silicon film either partly or wholly, by applying heat treatment to the amorphous silicon film while maintaining the catalyst element or the compound containing the same into contact with the film; and further accelerating the crystallization by irradiating a laser beam or an intense light equivalent thereto.
In this manner, a crystalline silicon film having an extremely high crystallinity can be obtained.
The catalyst element for accelerating the crystallization can be introduced most effectively by coating the surface of the amorphous silicon film with a solution containing the catalyst element.
Particularly in the present invention, the catalyst element must be introduced to the amorphous silicon film by bringing the elements into contact with the surface of the amorphous silicon film. This is the key in controlling the quantity of the catalyst to be introduced into the film.
The catalyst element may be introduced either on the upper surface or the lower surface of the amorphous silicon film. In case of introducing the catalyst element in the upper surface of the amorphous silicon film, a solution containing the catalyst element may be applied to the surface of the amorphous silicon film. In case of introducing the catalyst elements into the lower surface of the amorphous silicon film, the solution may be applied to the surface of the base film before depositing the amorphous silicon film, so that the catalyst element may be maintained on the surface of the base film.
The present invention is characterized in that it provides a semiconductor device having an active region containing at least one electrical junction such as a PN, PI, or NI by using the crystallized crystalline silicon film. The semiconductor device according to the present invention include a thin film transistor (TFT), a diode, an optical sensor, etc. Furthermore, a resistor or a capacitor can be formed by using the present invention.
The constitution of the present invention is basically advantageous in the following points:
(a) the concentration of the catalyst element in the solution can be precisely controlled in advance, enabling a crystalline silicon with higher crystallinity and less catalyst element;
(b) the amount of catalyst element to be introduced into amorphous silicon can be controlled in accordance with the concentration of the catalyst element in the solution so long as the surface of the amorphous silicon film be in contact with the solution;
(c) the catalyst element can be introduced at a minimum possible concentration, because the catalyst element adsorbed by the surface of the amorphous silicon film principally contributes to the crystallization; and
(d) a crystalline silicon film with favorable crystallinity can be obtained without using a high temperature process.
The solution containing an element which accelerates the crystallization of an amorphous silicon film can be applied in the form of, for example, an aqueous solution or a solution of an organic solvent. The term xe2x80x9csolutionxe2x80x9d as referred herein signifies a solution containing the element in the form of a compound, or a solvent in which the catalyst element is simply dispersed.
A polar solvent such as water, an alcohol, an acid, or ammonia can be used as the solvent for use in the present invention.
In case of using nickel as the catalyst and adding nickel into the polar solvent, nickel is introduced in the form of a compound. More specifically, it may be selected from a group of representative nickel compounds, i.e., nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate, nickel acetylacetonate, nickel 4-cyclohexylbutyrate, nickel oxide, and nickel hydroxide.
Otherwise, a non-polar solvent can be used in the solution containing the catalyst element. For example, a solvent selected from benzene, toluene, xylene, carbon tetrachloride, chloroform, ether, trichloroethylene, and Fleon can be used as well.
In this case, nickel is introduced in the solution in the form of a nickel compound. Representative compounds to be mentioned include nickel acetylacetonate and nickel 2-ethyl-hexanoate.
It is also useful to add a surface active agent into the solution containing the catalyst element. The surfactant increases the adhesion strength of the solution and controls the adsorptivity. The surfactant may be applied previously to the surface of the substrate onto which the amorphous silicon is deposited.
In case nickel simple substance is used as the catalyst element, it may be dissolved into an acid to provide a solution.
The description above is for a case nickel is dissolved completely in a solution. Nickel need not be completely dissolved in a solution, and other materials, such as an emulsion comprising nickel simple substance or a nickel compound in the form of a powder dispersed in a dispersant may be used as well. It is also possible to use a solution designed for forming an oxide film. Specifically mentioned as the solution is OCD (Ohka Diffusion Source) manufactured by Tokyo Ohka Kogyo Co., Ltd. A silicon oxide film can be easily obtained by applying OCD solution to the desired surface, and baking it at about 200xc2x0 C. An impurity can be freely added into the solution. Accordingly, OCD can be utilized in the process of the present invention. In such a case, the catalyst element is added into the oxide film, and the oxide film is then provided in contact with the amorphous silicon film. Then, the catalyst element can be diffused in the amorphous silicon film by heating the oxide film in a temperature range of from 350 to 400xc2x0 C. The resulting amorphous silicon film is then subjected to heat treatment for crystallization after the oxide film is removed. The heat treatment for the crystallization is effected in a temperature range of from 450 to 600xc2x0 C., for instance, at 550xc2x0 C. for about 4 hours.
The same as those mentioned in the foregoing is applied to the case in which a catalytic element other than nickel is used.
In case an aqueous solution containing a polar solvent such as water and nickel as the catalyst element for accelerating the crystallization of the amorphous silicon film is used, the aqueous solution is sometimes repelled by the amorphous silicon film if it were to be applied directly. This can be prevented from occurring by first forming a thin oxide film 100 xc3x85 or less in thickness on the surface of the amorphous silicon film, and then uniformly applying the solution containing the catalyst element to the surface of the resulting oxide film. Otherwise, the wettability of the amorphous silicon film can be improved by adding a surfactant and the like to the solution.
A toluene solution containing nickel 2-ethylhexanoate using the non-polar solvent can be directly applied to the surface of the amorphous silicon film. It is also effective to previously add an adhesive commonly used in coating a resist. However, the solution applied in an excessive amount reversely interferes the intrusion of the catalyst elements into the amorphous silicon film. Thus, the application of the solution to the surface of the amorphous silicon film must be performed with great care.
The amount of the catalyst element to be contained in the solution depends on the kind of the solution, however, roughly speaking, the amount of nickel by weight is from 1 to 200 ppm, and preferably, from 1 to 50 ppm. The concentration is determined based on the nickel concentration or the resistance against hydrofluoric acid of the film upon completion of the crystallization.
The crystallinity of the silicon film crystallized by heat-treatment can be further improved by irradiating a laser beam after the heat treatment. In case crystallization is partially effected by heat treatment, crystals can be grown from the heat-treated portion by irradiating a laser beam to obtain a state further improved in crystallinity.
A laser operated in pulsed mode can be used for the annealing above. For instance, useful lasers include excimer lasers such as a KrF laser emitting light at a wavelength of 248 nm, an XeCl laser (308 nm in wavelength), an XeF laser (351 nm or 353 nm in wavelength), an ArF laser (193 nm in wavelength), or a XeF laser (483 nm in wavelength). Furthermore, the lasers may be of a discharge excitation type, an X-ray excitation type, a light excitation type, a microwave excitation type, an electron beam excitation type, etc. The laser is preferably operated in long pulses with intervals in a range of from 10 to 100 xcexcs. By thus increasing the pulse interval, the silicon film can be kept molten for a longer duration to result in a silicon film improved in crystallinity.
In case the catalyst element is introduced in a small quantity, for instance, the crystallization occurs in minute spot-like regions. When viewed as a whole, this state can be regarded as a state in which crystalline components are mixed with amorphous components. By irradiating a laser beam to such a state, crystals can be allowed to grow from crystal nuclei present in the crystalline components. Thus, a silicon film improved in crystallinity can be obtained. In other words, small crystal grains can be grown into coarser crystals. Accordingly, the improvement in crystallinity by the irradiation of a laser beam becomes particularly distinct in case of an incompletely crystallized silicon film.
Otherwise, an intense light, particularly an infrared rays, can be irradiated in the place of a laser beam. Infrared rays is hardly absorbed by glass, but is readily absorbed by a thin film silicon. Accordingly, thin film silicon can be heated selectively without heating the glass substrate. This process of irradiating the infrared light for a short period of time is known as rapid thermal annealing (RTA) or rapid thermal process (RTP).
In the process according to the present invention, heat treatment is effected after accelerating the crystallization by irradiating a laser beam. The heat treatment can be performed under the same conditions as those employed in crystallizing the amorphous silicon film. As a matter of course, the heat treatment conditions need not be exactly the same as those employed in the previous heat treatment so long as it is effected at a temperature of 400xc2x0 C. or higher.
By thus performing heat treatment after irradiating a laser beam or an intense light, the defects in the crystalline silicon film can be considerably reduced. FIG. 8 shows the results obtained by electron spin resonance spectroscopy (ESR), i.e., the spin density of a crystalline silicon film fabricated under the conditions described in the conditions of sample preparation. The temperature and the duration of heat treatment are shown in the sample preparation conditions. In the conditions, LC represents the irradiation of a laser beam. All samples, except for the one denoted as xe2x80x9ccontaining no Nixe2x80x9d, are crystallized using nickel as the catalyst element. The xe2x80x9cg valuexe2x80x9d is an index representing the position of the spectrum, and a g value of 2.0055 corresponds to a spectrum attributed to a dangling bond. Thus, the spin density shown in FIG. 8 can be interpreted as such corresponding to the dangling bonds that are present in the film.
Referring to FIG. 8, the spin density is lowest for sample 4. This signifies that sample 4 has least dangling bonds, i.e., least defects and density of states. By comparing sample 3 with sample 4, for instance, it can be seen that the spin density is higher for sample 3 by one digit. It can be understood therefrom that the defects and density of states inside a crystalline silicon film can be lowered by one digit or more by adding a step of heat treatment after laser irradiation.
By further comparing sample 2 with sample 3 in FIG. 8, no considerable change occurs in spin density by irradiating a laser beam. That is, the irradiation of a laser beam has almost no effect in reducing the defects in the film. However, according to the analyses based on transmission electron micrphotographs and the like, the irradiation of a laser beam is extremely effective for accelerating the crystallization. Accordingly, it can be understood that the crystallinity of a crystalline silicon film once crystallized by heating can be enhanced most effectively by irradiating a laser beam, and that the application of a heat treatment again to the film whose crystallinity had been improved is particularly effective for reducing the defects within the film. In this manner, a silicon film having a high crystallinity and a low density of defects can be obtained.
In the process according to the present invention, crystal growth can be allowed to occur selectively by applying a solution containing a catalyst element to selected portions of the film. Particularly in case the solution is applied selectively, crystal growth proceeds from the solution-coated region towards the region having no solution coated thereto in a direction approximately parallel to the surface of the silicon film. The direction approximately in parallel with the surface of the silicon film is referred to hereinafter as xe2x80x9ca region of lateral crystal growthxe2x80x9d.
This region of lateral crystal growth is confirmed to contain catalyst elements at a low concentration. It is advantageous to use a crystalline silicon film for the active region of a semiconductor device, however, in general, the active layer region preferably contain impurities at a concentration as low as possible. In this context, the use of this region of lateral crystal growth, is particularly preferred for forming the active region of a semiconductor device.
In the present invention, particularly distinct effect is expected in case nickel is used as the catalyst element. Other useful catalyst elements are, preferably, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, and Au.
Also useful are the elements In, Sn, P, As, and Sb.
Furthermore applicable are the elements belonging to the Groups VIII, IIIb, IVb, and Vb of the periodic table.
The catalyst elements can be introduced not only by using a solution based on, for instance, an aqueous solution or an alcohol, but by widely using a substance containing the catalyst element. For instance, a metallic compound or an oxide containing the catalyst element can be used as well.
To further increase the degree of crystallization, the step of irradiating a laser beam or an intense light can be effected repeatedly for at least twice by taking turns with the step of heat treatment for decreasing the defects within the film.
By using an element of an intrusive type that accelerates crystallization, an amorphous silicon film can be crystallized at a low temperature, and yet, rapidly. More specifically, the crystallization of an amorphous silicon film can be accomplished, for the first time, by heating the film at a temperature as low as 550xc2x0 C. and in a duration as short as about 4 hours. Furthermore, because an intrusive element accelerates the crystallization by diffusing itself within the silicon film. Accordingly, a crystalline silicon film having no distinct grain boundaries can be obtained unlike from those obtained by crystal growth from crystal nuclei.
By furthermore irradiating a laser beam or an intense light to the crystalline silicon film obtained by crystallization through heating with a catalyst element incorporated therein, a highly crystalline silicon film reduced in the concentration of defects can be obtained.
The defects that are present in the film cannot be reduced by irradiating a laser beam. Moreover, a laser beam irradiated to the surface of a silicon film brings about instantaneously a molten state as to generate a stress inside the film. The stress newly induces a defect. Accordingly, by applying heat treatment to the film, the stress can be relaxed to reduce the defects, and thereby a crystalline silicon film having excellent electrical characteristics can be obtained.